A. Introduction
 

For maximum effectiveness, the first missile fired at a target should strike that target. Cost, size, and the necessity for surprise prohibit the firing of ranging shots (as was done with gun fire).

To strike the target on the first shot, the trajectory of the missile must be accurately controlled. This control is necessary because forces, natural or otherwise, can cause the missile to deviate from its predetermined course. Even though it functions perfectly, a missile without accurate guidance may miss a selected fixed target by several miles. Moving targets can take evasive action; without guidance, the missile would be unable to compensate for this action. Therefore, an accurate, fast-acting guidance system is of prime importance.

7A2. Definitions

The various systems of missile guidance were discussed briefly in chapter 6. This chapter deals with COMMAND GUIDANCE. The name means that intelligence (commands) is transmitted from an outside source while the missile is in flight. Current missiles controlled by command guidance include Regulus, Bullpup, and Nike.

A command guidance system incorporates two links between the missile and the control point.

One, an INFORMATION LINK, enables the control point to determine the position of the missile; the other, the COMMAND LINK, makes it possible for the control point to correct any deviations from the desired path.

7A3. Purpose and applications

The purpose of any guidance system is to secure direct hits on a selected target. Perfect performance is difficult to obtain because of natural disturbances and, in wartime, enemy countermeasures. However, because command guidance makes it possible to change the flight path of the missile by signals from the control

7A4. Basic principles

When command guidance is used, a ground, shipboard, or airborne station determines the position of the missile by radar tracking equipment or other means. It determines the error, if any, between the actual position of the missile and the desired position. It then sends out control impulses (commands) to bring the missile to the desired course.

If the flight path is long, and a large part of the path is over friendly territory or waters, several stations might track the missile as it comes into their range. These stations would then send commands to the missile to correct any deviations from the desired course.

7A5. Information links

The use of command guidance requires an accurate knowledge of the missile position, since all guidance comes from outside the missile. This knowledge is obtained through information links. The accuracy and dependability of the information link determines to a great extent the over-all accuracy of the complete system.

The information link enables the control point to determine the amount of error existing between the actual position of the missile and the desired position. Once this is known, correction signals can be sent to the missile.

Information links may use optical or electronic observation methods.

OPTICAL OBSERVATION. The optical, or visual, command guidance system has limited value, since the missile must always be visible from the command station. Such a system might use the unaided eye, telescopes, or optical rangefinders. But these devices are not

ELECTRONIC OBSERVATION. Much effort has been expended to develop an accurate and dependable electronic information link. A number of electronic systems have been designed and tested. The limitations of each system have been determined, and continuing efforts are being made to improve the most promising systems. Some electronic information links will be listed here, and more complete information on individual systems will be given later in this chapter.

7A6. Command links

The equipment used to send commands to a missile may be compared to a radiotelephone circuit between a piloted plane and a ground station. Instead of voice communications, the

TRANSMITTERS. Early target drone command transmitters were simple one-tube units that sent out a pulse when keyed by the operator. This system made it possible to control the rudder. But to control engine speed and altitude, additional transmitters tuned to other frequencies were required. As a result, the system became so large and complex that it was unsuitable. Consequently, work was started on a simpler, more reliable transmitter that would reduce the number of radio frequency (RF) channels needed for command guidance. The result of this work is the modern command guidance transmitter, which is similar to any medium power PM (phase modulated) transmitter. A block diagram is shown in figure 7A1.

The output of the crystal oscillator is built up by RF amplifier stages. Some of these stages operate as frequency multipliers, but the output stage operates as a straight-through RF power amplifier.

TRANSMITTER MODULATION. The use of PM results in considerable saving of space, power, and cost, since modulation takes place at a low level and requires less audio power than does high level AM.

Modulation is in the form of tones that are generated by tone generators. Each generator may be keyed separately or in combination with others. The tone generator outputs are fed to an audio mixer circuit and, as a result of the mixing, a composite tone appears at the output of the mixer stage.

The composite tone is fed to an audio pre-emphasis network. This network builds up (emphasizes) the higher audio frequency components of the composite signal. This action is desirable because atmospheric noise usually consists of high frequency components. Pre-emphasis is used only on the high frequency tones, and thus causes the signal-to-noise ratio to remain more constant throughout the audio range.

As shown in figure 7A1, the composite tone from the pre-emphasis network is fed to the phase modulator stage, which is connected between the crystal oscillator and the first frequency multiplier stage.

In order to understand how phase modulation (PM) takes place it is necessary to remember that the frequency of an alternating current is determined by the rate at which its phase changes. If the phase of the current in a circuit is changed, there is an instantaneous frequency change during the time that the phase is being shifted. The amount of frequency change, or deviation, depends on how rapidly the phase shift is accomplished. It also depends on the amount of phase shift. In a properly operating PM system, the amount of phase shift is proportional to the instantaneous amplitude of the

The RF section of the transmitter operates continuously, but is modulated only when one or more of the tone generators are operated by the keyer section.

RECEIVER. In the beginning, receivers used for remote control were simple one-tube super-regenerative sets. A relay was connected in the plate circuit of the tube; when a signal was applied to the input of the tube, its plate current changed and operated the relay. The closing of the relay contacts activated another circuit which moved the control surfaces.

The disadvantage of this system is that separate receivers are required for each control function. In addition, the superregenerative receiver, in its most sensitive condition, is a low-powered transmitter that could interfere with other receivers in the missile.

But receiver development kept pace with transmitter development, and simple one-tube sets were replaced by superheterodyne receivers. As shown in figure 7A2, these sets are identical to standard frequency modulation (FM) receivers (PM can be picked up by an FM set) up through the discriminator stage. In an FM set, the discriminator stage takes the place of the second detector in an AM superheterodyne.

USE OF TONE CHANNELS. The discriminator output is fed to AF channel selectors and there is one receiver channel selector for each tone the transmitter may send.

The sections of an AF channel selector are shown in figure 7A3. A sharply tuned band-pass filter (one that passes certain frequencies better than others) is at the input of an amplifier stage.

The grid bias of this stage is adjusted so that plate current is cut off when no signal is being fed to the stage. When a signal is applied to the input of the stage, the effective grid bias is reduced to the point where plate current flows. The change in plate current operates the relay; its contacts close, and activate the missile control surfaces.

7A7. Types of command guidance

As mentioned previously, command guidance may be exercised by one or more ground stations, shipboard stations, or aircraft. The guidance point influences the type of command guidance used. Since all command systems are subject to enemy jamming of the control circuit, the closer the missile can be launched to the target the better. A shorter time required for the missile to travel from the launcher to the target means less time for the enemy to jam the controls.

TELEVISION GUIDANCE SYSTEM. Television command guidance is well suited for some missions where the control point is in a mother aircraft. The control aircraft can stay out of range of hostile antiaircraft defenses and yet launch the missile reasonably close to the target. Because the target is picked up by the missile camera before the missile is launched from the aircraft, the missile controller in the plane sees the target through the missile camera from the time the target is first picked up until the missile strikes. Because of the close range at the time of firing, the system is quite accurate; and because of the short time between launching and striking, there is less chance of enemy jamming. But this is essentially an optical system, and is not

RADIO AND RADAR COMMAND GUIDANCE. These two systems are much alike. Each is based on a transmitter at the control point, and a receiver in the missile. The transmitter sends out a carrier wave, which is modulated in accordance with the command signals. The receiver interprets the modulation so that the missile can execute the transmitted commands. The two systems differ in two ways. First, radar operates at a higher frequency. Second, the radio transmitter usually sends out a continuous carrier wave, whereas the radar transmitter sends out its signals in the form of short pulses, with resting intervals between. Since both of these systems are treated in

HYPERBOLIC GUIDANCE SYSTEM. A hyperbolic guidance system can be used for both long and short range missile guidance. This system will be described more fully at the end of this chapter. It consists of master and slave stations that send out low-frequency pulses at constant intervals. The slave station is triggered by the master station, and sends out its pulses a few microseconds after the master pulse is transmitted. These pulses are picked up by receivers in the missile and fed to an automatic computer in the missile. The computer then establishes the missile position by an imaginary line of position set up by the master and slave stations.

A radio command system contains a means of accurately determining the missile position in relation to the control station, the target, and the desired trajectory. A computer is usually used to determine the error between the actual missile position and the desired position. A command transmitter is located at the control point, and a receiver is contained in the missile. The receiver activates the missile control circuits when it receives command signals from the transmitter. This equipment makes it possible to follow the missile' s flight and correct for errors which would cause a miss.

7B2. Applications

Radio command guidance may be used to control missiles aimed at ground targets from surface sites or from aircraft. The controlled missile may be of the surface-to-surface type, surface-to-air type, air-to-surface type, or air-to-air type.

7B3. Limitations

The limitations of a radio command system are imposed by transmission conditions, distance, and enemy countermeasures. Early systems, which used AM tone modulation, had additional limitations. As an example, an

The use of PM (phase modulation) eliminated a large part of the voice interference, but manmade interference with PM characteristics could still affect the control system. This disadvantage was overcome by using coded combinations of tone channels. With this system, no control operation can take place unless the proper tones appear at the missile receiver in the correct order and spacing. The adoption of this control method practically eliminates the chance that an interfering signal will duplicate the control combination.

7B4. Launching station components

MISSILE COURSE COMPUTER. Ordinary forms of course determination require a large number of calculations, and considerable time. Since calculations are time consuming, and since speed is an absolute necessity, electronic course computers have been developed.

The computer, located at or near the launching site, performs two functions. First, it determines the course that should be followed by the missile during its flight to the target. It then compares this desired course

MISSILE TRACKING RADAR. When command guidance is used, the position of the missile in relation to the control point, desired course, and the target area must be known at all times.

Since radar can provide information as to range, elevation, and direction, it is well suited for short- and medium-range missile tracking. In general, missile-tracking radars use the same principles as search, fighter-director, and fire control radars.

Radar ranging is accomplished by time measurement. The range is found by measuring the elapsed time between the transmission of a pulse and the arrival of the echo reflected from the missile. Radar waves travel at the speed of light (186,000 miles per second). The distance to the target is found by multiplying the elapsed time by the speed of the radar wave and then dividing the result by two. The division by two is necessary because the elapsed time includes time out and time back, so that the actual time to the target is one-half the elapsed time.

The time sequence for the radar set is started in the timing generator. The trigger action of the timing generator controls the modulator section, which in turn produces the high-voltage output pulse.

The same trigger pulse is also sent to the range unit, and starts its time-measuring device. After a short, fixed delay, the range unit forms a range gate. The gate is developed by a voltage which is present during a relatively short part of the main time cycle. This voltage is applied to the gain control circuit of the receiver. When the range-gate voltage is present, the receiver gain is high; during the rest of the time cycle, the gain is very low. Thus, when the range gate is "open," signals picked up by the antenna will pass through the receiver; when the gate is "closed," they will not.

A single antenna is used for both transmitting and receiving. This requires some means for switching the antenna from the transmitter to the receiver, and then back to the transmitter again. The device usually used for this purpose is called a duplexer. The duplexer makes it possible to operate the transmitter and receiver simultaneously, but keeps the powerful transmitter signals from entering the receiver directly.

For missile tracking, a lobing or conical scanning system is used, because accurate angle data cannot be obtained from a single beam on the antenna axis. This type of scanning is described in chapter 8.

Video signals produced by the reflected signal from the missile may be used to modulate the display on a cathode ray tube. The method of modulating the display will depend on the type of indicator used in the radar set. Either the deflection or the intensity of the beam trace may be modulated.

(The student interested in information on basic radar principles is referred to Supplementary Readings in Fundamental Naval Electronics, parts I and II, NavPers 10808 and 10809.)

MISSILE PLOTTING SYSTEM. The use of radar for missile tracking makes it possible to obtain information on the missile's elevation, bearing, and horizontal range. This information may be plotted so that personnel controlling the missile will have a complete picture of the operation.

An example of a basic plotting system is shown in figure 7B1. The tracking radar is shown at the left of the drawing, and the plotting board at the right. The boom on the

The radar can provide only slant range, bearing, and elevation angle. The horizontal range data used to position the tracing pen trolley can be obtained from the product of the slant range and the cosine of the elevation angle. The elevation of the missile is the product of the slant range and the sine of the elevation angle (or of the horizontal range and the tangent of the elevation angle). Successive positions of the missile can be marked on the plotting chart at regular intervals, to provide an indication of the missile's course.

COMMAND TRANSMITTER. The transmitter used to send commands to the missile is usually a tone-modulated FM unit. This type of transmitter was discussed earlier in this chapter.

7B5. Missile components

The command guidance equipment components that are built into the missile will be determined by the guidance system being used. The most complex guidance system has a television camera, television transmitter, radio command receiver, and the tone filter equipment built into the missile.

A relatively simple guidance system, so far as total equipment in the missile is concerned,

The most widely used command guidance system uses a radar tracking unit and a radio command link. The missile contains an FM receiver and AF channel selectors.

7B6. Operation of a typical system

A typical command guidance system might be used to control a surface-to-surface missile fired by a ship against a fixed installation ashore. The missile, during the early part of its flight, would be tracked by radar aboard the firing vessel. Because the geographical location of the target and the firing vessel are both known, the required missile course can be accurately calculated. Information from the missile-tracking radar may be fed to a computer, or it may be plotted on a visual display, or both. When the tracking data indicates that the missile has turned from its calculated course, commands can be transmitted to turn it back.

If the target is at fairly short range, the firing vessel may control the missile throughout its entire flight. At longer ranges, the tracking and command functions may be transferred to an aircraft, or they may be transferred successively to one or more ships located nearer to the target.

There is great similarity between radio and radar command guidance systems. However, there are some differences which must be considered when a guidance system is being designed.

Most radar command guidance systems depend on "sampling" control, since it is not possible to control all of the missile functions at once. Each must take its turn in the control sequence. Consequently, after a given function has received a command, there will be a time delay before the next command is received. The length of this delay will depend on the number of functions to be controlled.

When radar is used for control, the fidelity or accuracy of control is limited by the allowable variations in pulse rate or amplitude. (Excessive variations will affect the tracking accuracy.) The accuracy of control is also limited by the ability of the missile equipment to measure these variations accurately.

There are several ways in which commands can be sent by radar. For example, the pulse repetition rate (PRR) of the radar may be frequency modulated in order to turn the missile in the desired direction. If the PRR is unmodulated, no control signal is sent to the missile. If the PRR is modulated so that it increases, the missile will turn in a certain direction; if the PRR decreases, the missile will turn in the opposite direction. Since the PRR can be varied by the modulation frequency, it is possible to make the amount of turn proportional to the deviation from the normal PRR, and thus obtain accurate control.

This system requires some form of multiplexing or switching control, so that operations take place in a definite sequence. As an example, there may be five possible operations and each may be controlled by a 1/100-second signal of the proper pulse rate. A complete set of control signals could then be sent every 1/20 second.

The control pulses may be coded in sequence so that each pulse controls a particular operation. As soon as a full set of operations is covered, the sequence starts over again. The pulses may be modulated either in amplitude or by their position on a time scale.

1. Dual pulses are sent, and the spacing between pulses is varied according to the desired control signal. Or single pulses are transmitted, and when a particular control signal is desired, double, triple, or quadruple pulses are sent.

2. Alternate pulses may be displaced ahead of or behind their normal position and the desired control signal is determined by the amount and direction of the displacement. This is known as the displaced-pulse method of control.

3. By varying the width of the radar pulse, each control signal may be determined by the pulse width.

4. The radar pulse may be amplitude modulated so that the frequency of the modulated pulse envelope will determine the control signal.

5. The pulse rate may be varied indiscrete steps, with each frequency representing a different operation, such as climb or dive, right or left, explode, or dump. The degree of any operation, such as the amount or rate of climb, may be determined by the number of repetitions of the signal or the length of time a particular signal is maintained.

The use of the tracking radar beam as a control medium results in economy of equipment, because the radio control transmitter is no longer needed.

As in any other communications equipment, the bandwidth of the modulated signal determines the amount of information that can be transmitted in a given time. A radar signal with a pulse repetition rate of two thousand cycles per second would limit the number of functions that could be controlled, as well as the rate at which the control signals could be changed. But for some missile systems this bandwith is adequate, because only a few missile functions are under control by command guidance. And, if the rate of signal change is not too great, there will be enough bandwidth to allow modulation of the radar beam with several signals simultaneously.

If there are missiles in the vicinity of the beam other than the one being tracked, there

Standard Loran was developed primarily for long range navigation over water. The system requires at least two transmitting stations-one a MASTER, the other a SLAVE. The stations are separated by a distance of several miles, and the geographic location of each station is accurately known.

The master station transmits a signal which is radiated in a circular pattern. When the signal reaches the slave station, or stations, it triggers the slave which then sends out a pulse that is also radiated in a circular pattern. The signals of all stations travel outward from their respective antennas as shown in figure 7D1. At any point, such as P in figure 7D1, the signals will have different times of arrival because of the distances traveled and the differences in transmitting time.

Figure 7D1.-Basic Loran system.

Any plane passing through the line A-B, in figure 7D1, intersects these hyperboloids in such a manner that, in this plane, there passes only one branch of a hyperbola that is characterized by a constant range difference. Thus, if the range difference is known, a hyperbolic line of position on that plane is defined.

If a second line of position from another pair of Loran stations is known, a fix in the plane is determined by the intersection of the two hyperbolas.

Charts are available that show the hyperbolic lines of position associated with pairs of Loran stations in various areas. By using these charts, a navigator, knowing the range difference by radio measurement, can select his lines of position to get a fix.

Some work has been done toward the use of hyperbolic lines of position for missile command guidance. Therefore the basic parts of such a system will be briefly described.

MASTER TRANSMITTER. The master transmitter is a conventional CW transmitter radiating about 100 kw of power on one of several frequencies between 1700 and 2000 kc. The output is a series of pulses of accurately timed length. The ground wave range over sea water is about 700 nautical miles in the

SLAVE TRANSMITTER. The slave transmitter is a duplicate of the master transmitter except that the slave station includes a receiver to pick up the transmission of the master station. A relay in the output of the receiver keys the slave transmitter, so that it sends out pulses of the same length as those sent by the master station. But there is a difference in the start and stop time of the pulses, due to the time it takes a signal to reach the slave station from the master station.

AUTOMATIC RECEIVER. The delay between the sending of the master pulse and the slave pulse ensures that the master station pulse will always be picked up first at any receiver located in the area serviced by the system. The pulse time differences can be measured by displaying the pulses on a cathode ray oscilloscope that is provided with a precisely timed sweep. A missile using this system would have to measure the time difference automatically; this requires a relatively complex receiver. Because the system has no human operator, it is not possible to read pulse time difference on a cathode ray tube. Instead, the receiver uses a phase-shifting mechanism to match the phase of the slave station signal with that of the master station signal. The amount of phase shift required to produce a phase match gives an accurate indication of the difference in range of the two stations.

SERVO LOOPS. Servo loops are used to drive the phase-matching mechanisms. Signals received from the master and the slave stations are sent to a mixer; when the two are exactly in phase, the mixer output will be at a maximum. The mixer output is used to drive the phase-shifting servos in the direction required to produce a maximum output. One

COMPUTER AND AUTOMATIC PILOT. The two time-difference measurements, as indicated by the magnitude of the phase shifts produced by the servo mechanisms, is fed to a computer. The computer uses this information to calculate the position of the missile. Since the position of the target is known, the computer can then calculate the course which the missile must take to reach this target. The computer sends this information to the automatic pilot, which holds the missile on the required course.

When a hyperbolic system is used, a change in course is not apparent until the new course has been held for some time. In other words, the system gives an indication of position, not of direction. This is valuable since it makes missile navigation independent of air currents, and course and speed derived from hyperbolic systems are ground course and ground speed.

7D2. FM Loran system

A frequency-modulation (FM) Loran system is similar in function to the one just described, but it uses a unique approach to eliminate uncertainty. In a three-station FM system the outputs of three transmitters, master and two slaves, are frequency modulated by a sine wave. These three transmitters, with different low-frequency carriers, are frequency modulated by the same AF signal so as to obtain identical modulation in frequency and phase for the three transmitters. The time for one cycle of the modulation frequency must be long enough to allow the RF signal to get to the maximum range of the system.

A pair of transmitted signals are compared by measuring the relative delay required to produce a phase match in their modulating signals. Phase matching is indicated by an output of maximum amplitude from the mixer. As in the system previously described, the

The range of any hyperbolic navigation system depends on the frequency of the radiation that is used as a carrier. Ultra-high frequencies can be used for short range guidance systems with good accuracy. However, they are not good at the longer ranges.

By using microwave frequencies, a small, highly directional antenna can be mounted on the missile without interfering with its aerodynamic characteristics. The directional characteristics of the antenna are narrow in the vertical plane and fairly wide in the horizontal plane. These directional characteristics decrease the possibility of enemy countermeasures by jamming.

It is necessary to discriminate against sky wave interference in synchronizing the ground stations. To ensure accuracy, the synchronizing pulse must be transmitted via a direct, constant path. There should be no variable factors such as SKIP EFFECT which would alter the transmission of synchronizing signals.

It is difficult to establish a condition in which these variable factors do not change the transmission characteristics of an ultrahigh frequency (UHF) system. This is especially true where the baseline between stations is longer than the line-of-sight distance. All UHF installations require a means of relaying the synchronizing signal without introducing variations or unpredictable delays.

7E2. Three-station system

The transmitting stations use precision timing signal generators to modulate RF transmitters. These transmitters use the same kind of tubes and circuitry as a radar transmitter operating in the same frequency range. They must have high power output to give a high signal-to-noise ratio near the limit of their effective range. For UHF, this is normally assumed to be line-of-sight.

7E3. Four-station system

A four-station UHF system has the advantages of both line-of-sight transmissions and long baseline systems. Two pairs of stations are used; each pair consists of one master and one slave operating on the same frequency, and they are properly synchronized.

The pairs of stations are separated by enough distance so that lines of position on the hyperbolic grid are more nearly at a right angle to one another in the intended target area. If a second similar system is superimposed on the two-station grid at nearly aright angle, the missile position can be accurately determined.

To set up this system, one pair of guidance base stations is used to give the bearing guidance hyperbola. One time-difference line of this pair is chosen so that it will cross the target area of the missile. It then serves as the desired track for the missile. The guidance system in the missile determines when the received signal pulses have the proper TIME SEPARATION to show ON COURSE. If the received signals do not have the desired time difference, the guidance equipment can determine whether the missile is right or left of the desired course. The error signal from the guidance section is sent to the control section which makes the corrections to bring the missile on course.

The second pair of guidance base stations is used to determine the range. A particular time-difference line of this system is calculated

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This output voltage is converted to another voltage which is proportional to the rate of change of the output voltage. This proportional

It should be kept in mind that command guidance systems are in a state of constant development, and that future systems may differ considerably from those described here.

A. Introduction
 

The previous chapter discussed various methods by which commands can be sent from a control point to a missile, to control the missile flight from launching point to target. Beam-rider guidance system is in some respects similar to command guidance. In both systems, target information is collected and analyzed by suitable devices at the launching site or other control point-rather than by devices within the missile. In both systems, the missile makes use of guidance signals transmitted from the control point.

But beam riding is not considered a form of command guidance. The principal difference is this: in a command system, guidance signals are specific commands, such as "steer right," or "steer left." But the transmitter of a beam-rider guidance system transmits only information, not commands. By projecting a narrow beam of radar energy, the transmitter at the control station indicates the direction of the target (or, in some systems, the direction of a calculated point of intercept). The guidance system within the missile must interpret the information contained in the radar beam, and then formulate its own steering commands. These commands operate to keep the missile as nearly as possible in the center

The beam-rider system is highly effective for use with short-range and medium-range surface-to-air and air-to-air missiles. For missiles of longer range, a beam-riding system may be used during the midcourse phase of flight, while the missile is still within effective range of the beam-transmitting radar. As it approaches the limit of beam-riding range, the missile may switch over to some other form of guidance.

8A2. Application to U.S. Navy missiles

The development of missile guidance systems with minimum susceptibility to enemy countermeasures, and with maximum probability of hitting the target, is the primary objective of the U. S. Navy missile program. The program is continuous and has a high priority rating. Two missiles, Terrior and Talos, developed under this program have been operational for some time. Both of these are surface-to-air missiles using beam-rider guidance. This chapter will give information of a general nature on guidance systems that might be used with missiles of this type. It should be kept in mind that security requirements prevent a detailed description of the guidance system of any specific missile.

The radar energy that forms the guidance beam is transmitted by an antenna at the control point. Radiated energy tends to spread out equally in all directions. But by mounting a suitable reflector behind the antenna, a large part of the radiated energy can be formed into a relatively narrow beam. A narrow beam can point out the target direction with sufficient accuracy for the missile to score a hit, and concentration of the radiated energy into a beam extends the effective range of the system.

Figure 8B1.-Comparison of radiation from a lamp and a radio antenna.

centimeters; the wavelength of light ranges from about three to seven ten-thousandths of a millimeter.

You are, of course, familiar with the use of polished reflectors to form beams of light. An automobile headlight is an example of this, although it produces a fairly wide beam. A spotlight produces a more narrow beam. The upper part of figure 8B2 represents the reflection of light by an "ideal" reflector. The emerging rays are parallel; the beam is no wider than the reflector itself, and it does not diverge. But an ideal reflector is hard to

Figure 8B2.-Use of reflectors to form beams of radiant energy.

achieve in practice. It must be a paraboloid of revolution-that is, the surface generated by a parabola rotated on its axis. It must be highly polished; its surface irregularities must be small compared with the wavelength of light. And the light source must be a single point, located at the focus of the paraboloid.

The lower part of figure 8B2 represents the reflection of radar waves. Again, the surface

It should be noted that a light RAY is simply a convention used in diagrams of optical instruments. Such rays do not exist in nature. They are imaginary lines that indicate the direction in which the wavefronts are moving. Although RADAR RAYS are not a familiar convention, they are used in figure 8B2 to show the direction in which the radar waves are moving.

Of course the lamp shown in figure 8B2 is radiating light in all directions. The light from the front surface, which does not strike the reflector, will be scattered widely. In some spotlights, the front surface of the lamp is shielded, so that the only rays that leave the spotlight are those that have been reflected. Such a spotlight produces a sharply defined beam, with little or no scattered light. The same effect is achieved in radar by directing the opening of the waveguide backward, toward the reflector.

But no radar can produce an ideal beam of parallel "rays." For one thing, the end of the waveguide is large, compared to the ideal point source. For another, a reflector of practical size is not sufficiently large compared with the wavelength of the radiated energy. A radar beam therefore diverges and forms a lobe, like the one in figure 8B3. The student should clearly understand that such a lobe is merely a convenient way of representing the beam on paper; it is in no sense a "picture" of the beam. Some of the radiated energy will be scattered outside the lobe. And the radiation does not end abruptly at a certain distance from the transmitter, as the diagram implies. The lobe, if it can be pictured in three dimensions, can be thought of as a surface, all parts of which receive an equal amount of energy. This can be considered the minimum energy that is useful for our purpose (missile guidance or target tracking). And the lobe in figure 8B3 is not drawn to scale. The diameter of the reflector is in the order of two feet; the length of the lobe may be from 20 to 50 miles. Its useful width may be four or five degrees. At any given distance from the transmitter, the signal is strongest along the axis of the lobe.

Figure 8B3.-How r-f energy is concentrated in a lobe.

8B2. Conical scanning

In a beam-rider guidance system, radar must accomplish two things; it must track the target, and it must guide the missile. It would be difficult to do either of these things with a simple lobe like the one in figure 8B3. For example, assume that a target is somewhere on the lobe axis, and that the receiver is detecting signals reflected from the target. If these reflected signals decrease in strength, it will be apparent that the target has flown off the axis, and that the beam must be moved to continue tracking. The beam might be moved by an operator who is tracking the target with an optical sight; but such tracking would be slow and inaccurate, and would be limited by conditions of visibility. An automatic tracking system requires that the beam SCAN, or search, the target area.

Again, assume that a missile is riding the axis of a simple beam. The strength of the signals it receives will gradually decrease as its distance from the transmitter increases. If the signal strength decreases suddenly, the missile will know that it is no longer on the axis of the lobe. But it will NOT know which way to turn to get back on the axis. A simple beam does not contain enough information for missile guidance.

By a suitable movement of either the wave-guide or the antenna it is possible to generate a conical scan pattern, as shown in figure 8B4. The axis of the radar lobe is made to sweep out a cone in space; the apex of this cone is, of course, at the transmitter. At any given distance from the transmitter, the path of the lobe axis is a circle. Within the useful range of the beam, the inner edge of the lobe at all times overlaps the axis of scan.

Now assume that we use a conically scanned beam for target tracking. If the target is on the scan axis, the strength of the reflected signals will remain constant (or change gradually as the range changes). But if the target is slightly off the axis, the amplitude of the

Figure 8B4.-Conical scanning pattern.

reflected signals will change rapidly and periodically. For example, if the target is ABOVE the scan axis, the reflected signals will be of

When a conically scanned radar beam is used for missile guidance, the desired path of the missile is not along the axis of the beam, but along the axis of scan. Later in this chapter, we will show how the missile is able to guide itself along this axis.

Two types of beam-rider system are possible. In the simplest type, a single radar is used for both target tracking and missile guidance. In the other, one radar is used for tracking, while another generates the guidance beam. We will discuss the one-radar system, then point out briefly how the two-radar system differs.

8C2. One-radar system

In a one-radar system, the guidance beam is always pointed directly at the target, since the same beam is used for tracking. Two or more missiles can be in flight at the same time (toward the same target). The traffic handling capacity of the system is limited only by mutual interference between missiles in the beam. Once a missile has entered the beam path, no further operations are necessary at the launching site, except to maintain target tracking.

One factor must always be considered when an offensive weapon is used. That is, the enemy will always try to find countermeasures that will enable him to offset, or completely nullify, the effectiveness of the weapon. Some attempted countermeasures are fairly easy to overcome; others may be highly effective.

Another form of countermeasure might be an enemy radar set working on the same frequency as the guidance radar. This type of interference is called "jamming." The nature of the beam-rider guidance system gives good anti-jamming characteristics because the beam is narrow and directional. The missile carries its receiving antennas on its after end-often on its rear airfoils. These antennas are also directional; they are most sensitive to signals originating behind the missile, and relatively insensitive to signals originating in front. To effectively jam the guidance beam, the jamming transmitter must get behind the missile. Thus a jamming transmitter would be of little value as a defensive measure for a target aircraft, because once the target gets behind a given missile, it has already successfully evaded that missile.

It is also possible to transmit the guidance beam as a series of pulses having a definite,

Beam-rider guidance is used by both air-to-air and surface-to-air missiles. In neither application is the missile actually in the guidance beam at the instant of launching, and the problem of getting it there must be solved. For air-launched missiles, this is relatively easy; the missiles are carried beneath the wings of the aircraft, fairly close to the guidance radar. And they are fired directly forward; in most situations this is toward the target, and thus parallel to the guidance beam.

But when a surface-to-air missile is launched from the deck of a ship, the "capture" problem is more complex. The missile may be trained at almost any angle (except into the ship's structure). Because the blast of hot gases from the missile booster is deflected along the deck at the time of launching, a large area around the launcher must be kept clear. The guidance radar must therefore be located at some distance from the launcher. The missile cannot be launched directly toward the target, on a course parallel with the guidance beam. Instead, it must be launched in such a direction that it will CROSS the guidance beam a few seconds after launching. It will then turn toward the target, after it has been captured by the beam.

But because the guidance beam is narrow, merely aiming the missile to cross it is not enough to ensure capture. To make capture more certain, a broad CAPTURE BEAM (fig. 8C1) is superimposed on the narrow guidance beam. Because the energy in the capture beam is spread out over a large area, its effective range is short.

During the launching phase of missile flight, the control surfaces are locked and the guidance system is inoperative. The booster propels the missile in a direction calculated to place it within the capture beam. When the booster drops away, the control surfaces are unlocked and the guidance system takes over. The missile receiver is tuned to respond to the capture beam, and to seek its axis. In so doing, it turns itself toward the target and aligns itself in the guidance beam, which has

Figure 8C1.-Capture beam and guidance beam.

the same scan axis as the capture beam. After a preset interval, a timing device within the missile changes the receiver tuning. The missile will then reject signals from the capture beam, and respond only to those in the guidance beam, which has a different carrier frequency.

The single-radar beam-rider system, because it uses only one radar instead of two, has the advantage of simplicity. But the use of a single radar results in a serious problem. Remember that the guidance beam is also the tracking beam, and must therefore be pointed at the target throughout the missile flight. Except in one special case-when the target is flying directly toward the transmitter-the radar must be trained in order to follow the target. For a nearby, high-speed, crossing target, the angular rate of train will be high. The missile course, therefore, cannot be a straight line. The missile must constantly move sideways in order to stay in the beam. While the missile is relatively close to the transmitter, its lateral rate is small. But, as the missile approaches the target, the same angular rate of train will require increasing lateral acceleration of the missile.

Figure 8C2 illustrates this problem by showing three successive positions of the target and the missile. In this example, the beam

Figure 8C2.-Lateral movement of missile.

is trained to the left at an almost uniform rate. The missile, in order to stay in the beam, must accelerate to the left at a rapidly increasing rate. In the extreme case shown in the figure, the missile as it nears the target, must follow a path almost at a right angle to the beam. Even with its control surfaces in their extreme positions, the missile would probably be unable to turn at the required rate. Thus a one-radar beam rider might be useful against approaching targets, but ineffective against high-speed crossing targets.

8C3. Two-radar system

The two-radar beam-riding system uses one radar to track the target and a second radar to guide the missile. A computer is used between the two, and the guidance radar is controlled by the computer. The computing system uses information from the tracking radar to determine the trajectory necessary to ensure a collision between the missile and the target. Because the same radar beam is no longer used for both tracking and guidance, the missile need not follow a line-of-sight path, as was the case with a one-radar system.

The two-radar beam-guidance system is more complex insofar as ground equipment is concerned, because of the addition of a computer and a second radar. The equipment in the missile is the same for either system.

From the information that has been given, it may be seen that the computer is an important part of a two-radar guidance system. The computer takes information-speed, range, and course-from the tracking radar. From this information, it computes the course that must be followed by the missile. Since the computer receives information constantly, it can and does alter the missile course as necessary to offset evasive action or changes in course by the target. The output of the computer controls the direction of the guidance radar antenna. Required course changes are instantly transmitted to the missile by pointing the guidance beam toward the new point of intercept.

As we mentioned earlier, lateral acceleration presents a serious problem when a one-radar guidance system is used, because the missile course is changed by the angular movement of the tracking beam. This problem is not present in a two-radar guidance system because the missile course is directed toward a collision point, rather than toward the constantly changing position of the target. Because course information is continuously fed to the missile guidance radar, the missile trajectory is straight or only slightly curved from the launching point to the target.

There are several important components, other than the missile and radar, in a complete

8D2. Launching station components

The target is usually picked up at long range by a search radar. When the target is identified, the tracking radar takes over the job of following it, and determines its direction, speed, and range. This information is converted rapidly into useable form by the computer.

Because the computer is given the course, speed, bearing, elevation, and instantaneous range of the target, it can calculate the position of the target at any future time, assuming that it does not change course or speed. The computer is also given the average speed of the missile. With this information, it is able to determine the direction in which the missile must be launched to intercept the target. It is unlikely that a missile will be fired as soon as the tracking radar acquires the target. The target range is constantly changing, and the target may change course or speed as well. The computer must therefore produce a continuous solution to a continuously changing problem. At any given instant, the computer output provides the correct solution to the problem as it exists at that instant.

In a two-radar system, the computer continues to calculate the missile course after the missile has been launched, and until the target has been destroyed. Through servo mechanisms, it turns the control radar in the proper direction. In a one-radar system, the computer output is used to train and elevate the missile launchers in such a direction that

8D3. Missile components

The receiving antenna in the missile is a very important part of its electronic installation. Through it must come all guidance signals from the control radar. There are several difficulties in determining the optimum location of an antenna on a missile. First, the antenna must not interfere with the aerodynamic stability of the missile. Second, it must be located at a point where it will not be damaged by the rapid acceleration as the missile is launched, and where wind will not tear it loose. Finally, the antenna must be located where it can effectively pick up the signals of the guidance beam. The antenna location that has been found most satisfactory is on the missile tail surfaces.

The missile antenna is highly directional, and most sensitive to signals received from behind the missile. The roll-control system of the missile keeps it stabilized so that the antenna polarization remains constant.

The guidance signals picked up by the missile antenna are fed to a receiver. After the signals are amplified and demodulated by the receiver, they are fed to a computer. If the missile is off the scan axis of the guidance beam, the computer will determine both the direction and the magnitude of the error. It will then give the control system the commands required to bring the missile back onto the scan axis.

Earlier sections of this chapter have shown that a number of components are required to complete a beam guidance system. Each component must function properly if the missile is to destroy the target. But no system can be expected to operate beyond its natural limitations.

R_E = sqrt(2H_t) + sqrt(V2H_r)

It can be seen that raising the height of either antenna will increase the effective range. Thus it appears that a missile, because of the altitude at which it travels, can be controlled at extremely long range. But this is not true. The transmitter power necessary to deliver a satisfactory control signal increases rapidly with distance. Therefore, for long range missiles, a single beam-rider guidance system would be unsatisfactory. But these limitations can be overcome by using beam-rider guidance during the first part of the missile flight, then switching to a different guidance system before the missile flies beyond control of the radar beam.

8E2. Tracking radar

We have mentioned that the tracking radar furnishes information as to the position of the target. All target position references are made with respect to the scan axis of the tracking lobe.

The amount of energy in the beam falls off rapidly at points away from the center of the lobe. Figure 8E1 shows the relative amounts of energy transmitted at various angles to one side of the lobe axis. Because of the variation in transmitted energy, there will be a corresponding variation in the strength of signals reflected by targets at various angular distances from the center of the lobe.

Figure 8E1.-Radiated energy variation.

As we mentioned earlier, the tracking system is automatic. After the tracking radar has acquired the target, tracking is maintained without the help of a human operator. But the action of the tracking system is monitored by an observer, who may take over and track the target manually if the automatic system fails.

The lower diagram shows the effect of a target above the scan axis of the beam. When the lobe is in its highest position, the target is directly on the lobe axis, and the height of the CRT pip is a maximum. When the lobe is in its lowest position the target is far off the lobe axis; its reflected signal will be much weaker, and the pip on the CRT correspondingly small. This indicates that the target is above the scan axis.

A second CRT indicates the relative strength of the reflected signals when the lobe is at its extreme left and extreme right positions. In an emergency, the operator can track the target manually by moving the radar so as to keep the pairs of pips of equal height on both CRT's.

8E3. Control radar

A block diagram showing the sections of a beam-rider guidance system is shown in figure 8E3. The guidance beam pattern is formed by the antenna of the guidance radar. Transmitter sections are shown in the dashed squares of figure 8E3. The sections in solid lines are in the missile.

As explained earlier, a conical scan is one in which the lobe axis of the radar beam is moved so as to generate a cone. The vertex of this cone is at the antenna. It is possible to produce a conical scan by any of several methods.

The proper length for a dipole IN FREE SPACE can be determined from the formula:

Length (feet) = 492 / Frequency(mcs)

However, free space conditions do not exist in actual antenna installations, and the most

The formula shows that a dipole antenna for 450 mcs would be about 13 inches long. Therefore, the physical size of a highly directional array (antenna and reflector) for that frequency would be small enough for easy mounting and rotation.

When we speak of rotating a dipole, we do not mean that the antenna is turned or rotated about its center. If this were done, the antenna polarization would change as the antenna turned. Polarization would be vertical when

Figure 8E3.-Block diagram of beam-rider system.

the antenna was perpendicular to the ground, and horizontal when the antenna was parallel to the ground. Should this condition exist, control of the missile would be erratic because the anti-roll controls on the missile are designed to maintain constant polarization of the missile antenna.

Figure 8E4.-How a dipole is rotated.

ROTATING REFLECTOR. When the antenna rotates, as described above, the relative motion between antenna and reflector produces a conically scanned beam. It is apparent that the same relative motion can be produced by using a stationary antenna and rotating the reflector about a point off its axis.

NUTATING WAVEGUIDE. A waveguide is a metal pipe, usually rectangular in cross- section, which is used to conduct the r-f energy from the transmitter to the antenna. The open end of the waveguide faces the concave side of the reflector, and the r-f energy it emits is bounced from the reflector surface.

A conical scan can be generated by nutation of the waveguide. In this process, the axis of the waveguide itself is moved through a small conical pattern. Figure 8E5 is an attempt to represent this three-dimensional movement in a two-dimensional diagram. Nutation is difficult to describe in words, but easy to

Figure 8E5.-Nutation of the waveguide.

8E4. Missile response

A beam-riding missile must guide itself to the target by following the scan axis of its guidance beam. The only guidance information available to the missile is that contained in the beam. From this information, the missile guidance system must determine three things: (1) whether or not the missile is on the beam axis; (2) if not, how far it is off the axis, and (3) which way to go to get back on the axis. The first and third requirements are fairly obvious. The necessity for measuring the AMOUNT of error is less apparent.

During the early stages of guided missile development, one of the more serious problems was "overshooting." When a missile moved off course, and received a signal intended to correct the error, it would turn back toward the course, but overshoot and go too far in the opposite direction. This effect was caused by the lag in the response of the control system to guidance signals, and in the response of the missile itself to movement of its control surfaces. For practical purposes, this problem has been solved by the use of error signals proportional in magnitude to the errors they are intended to correct. Thus if a missile is far from the beam axis it will generate a large error signal, and its control surfaces will be turned through a relatively large angle. But, as the missile moves back toward the beam axis, its error signal steadily decreases, and the angle of its control surfaces is decreased accordingly. At the instant the missile reaches the beam axis, its control surfaces will (in theory at least) have reached their neutral position, and overshooting will be prevented.

Now let us see how the missile determines whether or not it is on the scan axis. Figure 8E6 represents a missile below the scan axis of the guidance beam. The path of the lobe axis is a circle. And the amplitude of the

Figure 8E6.-Missile below the beam axis.

radar signal is at a maximum along the axis of the lobe. As the lobe axis sweeps near the missile, the signal will be strong; as it sweeps away from the missile, the signal will decrease. To the missile, it will appear that the signal strength is regularly changing in amplitude, at the same frequency as that of the scan cycle.

The missile receiver is provided with a detector, which eliminates the r-f carrier frequency and produces a sine wave signal of the scan frequency. When this a-m signal is present, the missile knows that it is OFF the scan axis of the beam. When the a-m signal is absent, the missile knows that it is ON the axis. To see this clearly, look at figure 8E6 and imagine the missile on the scan axis. It is now at the same distance from the lobe axis throughout the scan cycle, and the amplitude of the r-f signal it receives remains constant.

From the AMPLITUDE of the a-m signal, the missile can determine how far it is from the scan axis. When the missile is on the axis, the amplitude of the a-m signal is zero, indicating zero error. If it is only a short distance from the scan axis, its distance from the lobe axis changes only slightly during the scan cycle. The a-m signal will thus be small, indicating a small error. Now, looking at figure 8E6, imagine the missile at some point on the circular path of the lobe axis. The variation in its distance from the lobe axis during the scan cycle is now at a maximum. The a-m signal will also be at a maximum,

(It is apparent that if the missile moves to a point OUTSIDE the circular path of the lobe axis, the error signal will decrease. But this does not happen in practice, unless the missile is defective. The guidance and control systems are too sensitive to allow so large an error to develop.)

How the missile determines the DIRECTION of its error can best be explained in two steps. Figure 8E7 shows an imaginary scanning system in which the lobe of radar energy, instead of sweeping out a cone, has only two positions-up or down. The two lobes are transmitted alternately. The figure shows the missile below the scan axis, near the axis of the lower lobe. The missile will receive signals from both lobes, but those from the lower lobe will be of greater amplitude. If we can provide the missile with some means for distinguishing between the two lobes, so that it can tell WHICH ONE has the stronger signal, it can determine the direction of its error. For example, if the missile in figure 8E7 can determine that it is the LOWER lobe that has the stronger signal, it will know that it must move up to get back on the scan axis.

Figure 8E7.-Two-lobe scanning system.

There are two fairly simple ways in which we can identify the two lobes so that the missile can distinguish between them. (We cannot, of course, make them of different amplitude, since a missile on the scan axis would then detect a false error signal.) Beam-rider radar transmission consists of an extremely

Thus the imaginary two-lobe scanning system could be used for guiding a beam rider in a vertical plane. If we add two additional lobes, each of which the missile can distinguish from the other two, it would also be possible to guide the missile to right or left. It should now be apparent that we can guide the missile in any direction by using a conical scan.

Look back at figure 8E6. Assume that we vary the signal frequency (either the carrier or the pulse rate) sinusoidally at the scan frequency. Assume that when the lobe is at its highest point, the signal frequency is at a maximum. As it moves around to the right side of its circular path, the signal frequency decreases to its average value. At the lowest position of the lobe, the signal frequency is at a minimum. It increases to average value as the lobe approaches the left side of its path, and to a maximum as it returns to its highest position. Thus the signal of the guidance beam is frequency modulated at the scan frequency. Note that the f-m signal is always present at the missile, regardless of whether it is on or off the scan axis.

The missile receiver is provided with an f-m section, the output of which is a sine wave that indicates the instantaneous position of the lobe in its scan cycle. The sine wave will have a maximum positive value when the signal frequency is maximum; it will pass through zero as the signal passes through its average frequency; it will reach its maximum negative value when the signal frequency is at a minimum.

The missile can determine the direction of its error by comparing the phase of the f-m signal with that of the a-m signal. Refer to figure 8E6 again; here the missile is directly below the scan axis. The signal will be strongest, and the a-m signal will reach its maximum positive value, as the lobe passes through its

Phase comparison is a fairly easy job for an electronic computer. The computer has been programmed to measure the phase relationship, to determine the direction in which the missile must move to return to the scan

To summarize: The guidance beam is conically scanned, and frequency modulated at the scan rate. If the missile detects an a-m signal, it will know that it is off the scan axis; if it detects no a-m signal, it will know that it is on the axis. The amplitude of the a-m signal indicates the size of the error. A large error will produce a large movement of the missile control surfaces. As the missile approaches the beam axis the error decreases, and the position of the control surfaces gradually returns to neutral, to prevent overshooting. The phase relation between the a-m and f-m signals indicates the direction of the error.

Every mechanical or electrical system has limitations that cannot be exceeded. When working with complex mechanisms, such as guided missiles, it is as important to know limitations as it is to know the capabilities. Unless the limitations are known, a costly missile might be wasted.

One important limitation is the maximum range at which reliable control can be maintained. We have mentioned line-of-sight limitation. Bear in mind that this statement does not mean that the missile must remain within range of vision. It does mean that control may be lost if the path between the missile and the guidance radar extends over the horizon or is blocked by hills or mountains.

Another limitation, previously mentioned, is transmitter power. In theory, at least, any amount of power can be generated. Radar systems through pulse techniques, make it

It should also be kept in mind that the radar beam increases in width and decreases in power as the range is extended, resulting in a decrease in both tracking and guidance accuracy at long ranges.

We have previously explained that countermeasures may decrease the effectiveness of an offensive weapon. The susceptibility of a guided missile to countermeasures is a limitation to its use. The effectiveness of countermeasure action can be greatly reduced by using coded-pulse modulation of the radar guidance beam.

A. Introduction
 

In previous chapters, we have discussed guidance systems that are designed to place and hold a missile on a collision path with its target. As we have previously explained, missile guidance can be divided into three phases: launching, intermediate or midcourse guidance, and terminal guidance. The proper functioning of the guidance system during the terminal phase, when the missile is rapidly approaching its target, is of extreme importance. A great deal of work has been done to develop extremely accurate equipment for use in terminal-phase guidance.

This chapter will discuss some of the homing systems that have been found to be effective for terminal guidance, as well as some systems that, in their present state of development, have serious limitations.

The expression HOMING GUIDANCE is used to describe a missile system that can "see" the target by some means, and then by sending commands to its own control surfaces, guide itself to the target. (Use of the word "see" in this context does not necessarily mean that an optical system is used. It simply means that the target is detected by one or more of the sensing systems that will be described later in this chapter).

9A2. Basic principles

Some homing guidance systems are based on use of the characteristics of the target itself as a means of attracting the missile. In other words the target becomes a lure, in much the same manner as a strong light attracts bugs at night. Just as certain lights attract more bugs than others, certain target characteristics provide more effective homing information than others. And some target characteristics are such that missiles depending on them for homing guidance are very susceptable to countermeasures.

Other homing systems illuminate the target by radar or other electromagnetic means, and use the signals reflected by the target for homing guidance.

The various homing guidance systems have been divided into PASSIVE, SEMIACTIVE, and

If the target emits the homing stimulus, the system used to detect the target and guide the missile to it is known as a PASSIVE HOMING guidance system. One such system uses radio broadcast waves from the target area as signals to home on.

If the target is illuminated by some source other than itself or equipment in the missile, the system is known as a SEMIACTIVE HOMING system. For example, the target might be illuminated by equipment at the missile launching station.

If the target is illuminated by equipment in the missile, the system is called an ACTIVE HOMING guidance system. An example is a system that uses a radar set in the missile to illuminate the target, and then uses the radar reflections from the target for missile guidance.

9A3. Types of missile response

When the control surfaces of the missile are activated by one of the guidance systems, the missile is showing response to the guidance system. A number of guidance systems have been developed to respond to a variety of signal sources. These sources are:

SOUND. If we go through the frequency spectrum from low to high, we can list systems in order of frequency and start in the audio (low) range. Sound has been used for guidance of naval torpedoes, which home on noise from the target ship's propellers. But a guidance system based on sound is limited in range. The missile or torpedo must use a carefully shielded sound pickup, so that it will not be affected by its own motor noise. And while the speed of a torpedo is low compared to the speed of sound, most guided missiles are supersonic. Because of these limitations, no current missile uses a guidance system based on sound.

RADIO. Most homing guidance systems use electromagnetic radiations. Radio waves are used in one passive homing system. Homing is accomplished by an automatic radio direction finder in the missile. The equipment is tuned to a station in the target area, and the

While it is possible to do a thorough job of confusing a radio homing guidance system, there is one possibility that cannot be overlooked. The enemy must use electromagnetic systems for communications and search, and these systems can be used as a source of guidance signals. Also, it is possible for subversive agents to plant small, hidden radio transmitters in target areas.

RADAR. Although radar can be used for all classes of homing guidance, it is best suited for the semiactive and active classes. At present, radar is the most effective source of information for homing guidance systems. It is not restricted by weather or visibility, but under some conditions it may be subject to jamming by enemy countermeasure equipment.

HEAT. One form of passive homing system uses heat as a source of target information. Another name applied to this system is INFRARED homing guidance. Heat generated by aircraft engines or rockets is difficult to shield. In addition, a heated path is left in the air for a short time after the target has passed, and an ultra-sensitive heat sensor can follow the heated path to the object. One present limitation is the sensitivity of sensor units. As sensor units of higher sensitivity become available, infrared homing guidance will become increasingly effective. Such systems will make it difficult to jam the homing circuits, or to decoy the missile away from the target.

LIGHT. A passive homing system could be designed to home on light given off by the target. But, like any optical system, this one would be limited by conditions of weather and visibility. And it would be highly susceptible to enemy countermeasures.

In command and beam-rider guidance, the missile is controlled from the launching site, or from some other point at a considerable distance from the target. But neither of these systems is very effective against moving targets, except at relatively short ranges. The reason is obvious. The closer the missile gets to the target, the farther it is from its control point. At long range, a very small angular error in target tracking, missile tracking, or beam riding could cause the missile to miss its target by a wide margin.

Sidewinder is a Navy missile that uses homing as its only source of guidance. It has been used very effectively at relatively short range. But homing systems are based on information radiated from, or reflected by, the target itself. For targets at intermediate ranges, such signals are extremely weak, and could be used only by missiles with powerful and heavy guidance equipment. At long range, such signals are entirely unavailable.

An answer to this problem lies in the use of a composite guidance system. In this system, the missile is guided during its intermediate phase by information transmitted from the launching site, or other friendly control point. During the terminal phase, it is guided by information from the target. For intermediate-range missiles, either command or beam-rider guidance is suitable during the midcourse phase. A long-range missile would depend either on preset or navigational guidance to bring it to the target vicinity. Missiles of both classes can switch over to homing guidance, based on infrared or radar radiations, as they enter the terminal phase. At intermediate range, the switchover is usually accomplished by radio command. At long range, it is controlled by a navigational device, or by some form of built-in programing system.

The U. S. Navy missile program makes use of composite guidance systems in several of its operational missiles. Talos is a beam rider during its midcourse phase, and switches to radar homing for terminal guidance. Other missiles, such as Sidewinder, Sparrow, Petrel, and Tartar, use homing guidance systems in one form or another for terminal guidance.

515354 O-59-11

As mentioned earlier in this chapter, passive homing systems can be used when the target itself radiates information. Therefore, all of the response systems, with the possible exception of radar, might be used. The exception to radar would not apply if a signal from the target could be picked up by a radar set in the missile. This system would be unreliable; the only source of target information would be under enemy control, and could be switched off at will. But missiles using such a system would have one distinct advantage: they would deny the enemy the use of his own radar.

9B2. Basic principles

The passive homing systems most widely used at present are based on infrared radiation from the target. The sensing mechanism is so designed that it can determine the direction from which the infrared radiation is received; the guidance system can then steer the missile in that direction. There are several ways in which the sensing device can be made to determine the direction of an infrared source. For example, two sensors could be mounted with a baffle between them, so that the one on the right can receive radiation from straight ahead, or from any point to the right of the missile axis. The other sensor will receive radiation coming from the straight ahead or from the other side of the axis. When both sensors receive the same amount of radiation, the target is directly ahead. If the radiation is stronger on one side, the target is obviously on that side. A second pair of ,sensors could be used for up-and-down steering.

9B3. Target characteristics

In passive homing, the target itself must provide all the necessary information for missile guidance. For this reason the characteristics of an individual target will determine which types of homing system can be used against it, and under what conditions they can be used.

If the target is fixed in location, and has some characteristic by which the missile can readily distinguish it from the surrounding area, the homing guidance problem is simplified. Figure 9B1 represents an air-to-surface or surface-to-surface missile, using a light-sensing guidance system to home on an industrial building. While such a missile might be useful in a surprise attack, industrial plants would certainly be blacked out during a war. A light-homing missile would then have no way to distinguish the target from its background. But infrared passive homing could be used in this application. And it would probably be more effective than light-homing, since the heat generated by an industrial plant can not be readily controlled.

Figure 9B2 represents a passive infrared homing missile attacking an aircraft. The Navy's Sidewinder uses this type of guidance. The tailpipe of a jet aircraft is a strong source of infrared radiation, which cannot be

A sound-homing system might also be used against a jet aircraft target, even though both target and missile are traveling faster than sound. Such a system might be used in a tail chase, provided the target does not maneuver radically. But you have probably observed that when a jet passes over at moderate altitude, the sound appears to come from a point at some distance behind the aircraft. A sound-homing missile would steer itself toward the source of sound, rather than toward the target itself. For an approaching or crossing target, the required trajectory would be too sharply curved for the missile to follow.

9B4. Missile components

When passive homing guidance is used, the missile must contain all of the equipment needed to pick up, process, and use the information given off by the target. The kind and amount of equipment required is determined to a large extent by the guidance system used, and by the characteristics of the target.

ANTENNA OR OTHER SENSOR. Since information given off by the target is to be used for guidance, some means must be provided to pick up the information. For electromagnetic systems, a conventional radio or radar antenna (streamlined into the missile) would be used.

A heat-sensing detector, rather than an antenna, is used with infrared homing guidance systems. One of the basic heat detectors is called a THERMOCOUPLE. When two dissimilar metals, such as iron and copper, are joined and heat applied to the junction of the two metals, a measurable voltage will be generated between them. Figure 9B4 shows a basic thermocouple.

The voltage difference between the two metals is quite small, but the sensitivity can be increased to a point where the thermocouple becomes useful as a detector of heat. The

The sensitivity of a thermopile heat detector can be further increased by mounting the thermopile at the focal point of a parabolic reflector. When this method is used, heat rays given off by the target are focused on the thermopile by the reflector.

Another type of heat detector is called a BOLOMETER. This device depends on the change of electrical resistance of a material when heated. In a simple type of bolometer, two thin strips of platinum are used to form two arms of a Wheatstone bridge. To increase the thermal sensitivity of the strips, each is blackened on one side. The heat to be measured is applied to one of the strips, and is absorbed by its blackened surface. As the strip absorbs heat, its resistance changes and unbalances the bridge. This unbalance causes

Figure 9B5 shows a modern bolometer; it consists of four nickel strips supported by phosphor bronze springs. These springs are supported by mounting bars, which have electrical connection leads attached to them. A silvered parabolic reflector (mirror) is used to focus infrared rays on the bolometer. The bridge unbalance current, produced as a result of resistance changes, is used to set in motion the other sections of the guidance system.

Figure 9B4.-A basic thermocouple.

Figure 9B5.-Modern bolometer.

Still another form of infrared detector is shown in figure 9B6. It is called a GOLAY DETECTOR. This detector is a miniature heat engine. The Golay heat cell operates on the principle that a pressure-volume change occurs in a gas when its temperature is changed. At the forward end of the cell is a metal chamber which encloses the gas. The front of the chamber is covered by a membrane, which acts as a receiving element. The back of the chamber is closed by a flexible mirror membrane. When radiant heat strikes the receiver, it raises the temperature of the gas in the chamber. The resulting increase in pressure distends the mirror membrane. Light from a small exciter lamp (fig. 9B6) is focused by a lens, and then passed through a grid of parallel lines. The light is then reflected by the mirror membrane, back onto the grid.

When the mirror membrane is not distended, it reflects the image of the open spaces in the grid back onto the opaque lines of the

Figure 9B6.-A Golay detector.

A part of the light that passes back through the grid is reflected downward by a diagonal mirror (shown at the lower right in fig. 9B6). This light is then picked up by a photoelectric cell. The output of the photo cell thus provides an indication of radiant heat entering the detector. The device is quite sensitive, since a small amount of mirror distortion produces a considerable change in photo cell illumination. The Golay detector has the most rapid response of any infrared detector, but it can operate only when radiant heat is received intermittently. For some guidance systems this factor makes the Golay detector useless; in others it causes no difficulty.

In the light-homing guidance systems, the pickup device or sensor is a photoelectric cell. The operation of this device is based on the fact that certain metallic substances emit electrons when they are exposed to light. Modern photoelectric cells are quite sensitive to light variations; but, because light is easily interrupted, the system is subject to interference. One type of photoelectric cell is shown in figure 9B7.

Figure 9B7.-A photoelectric cell.

ANTENNA OR SENSOR DRIVE. Previous chapters have described antenna scanning

Should the sensor temporarily lose sight of the target, a spiral or sawtooth scan, as shown in figure 9B8, could be used to find the target again. Notice that both types of scan cover a large area.

After the antenna or sensor has picked up the information, other equipment in the missile must convert the information into error signals, if the missile is off course. Before this can be done, however, there must be something to compare with the sensor signal.

REFERENCE UNIT. The comparison voltage is taken from the reference unit. This voltage may be o b t a in e d from an outside source, or it may be taken from recorded information that was put into the missile before launching. Actual operation of the missile guidance controls takes place only when an error signal is present. Note that the reference unit is connected to both the pitch and yaw comparators in the block diagram of figure 9B3.

SIGNAL CONVERTER. The output of the sensor unit is an extremely small voltage. This voltage is fed to a signal converter, which

COMPARATORS. The comparators are electronic calculators that rapidly compare reference and signal voltages and determine the difference (error), if any, between the two signals. It is possible for an error signal to be developed in the pitch comparator while no error signal is developed in the yaw comparator. Should this happen, the missile would be higher or lower than the desired trajectory. The output voltage from the pitch comparator is then fed to the missile automatic pilot.

AUTOPILOT. The automatic pilot, or autopilot, operates missile flight controls in much the same way as a human pilot operates airplane controls. The components making up the autopilot assembly have been described elsewhere in this text. In order to shift the flight controls, the autopilot must get "orders" from

In a semiactive homing system, the target is illuminated by some means outside the target or the missile. Normally, radar is used for this type of homing guidance, by sending a radar beam to the target. The beam is reflected from the target, and picked up by equipment in the missile. The radar transmitter might be located at a ground site, or it might be a mobile unit aboard a ship or aircraft.

9C2. Launching station components

In a semiactive homing guidance system, the launching station components are similar to those required for a beam-rider guidance system (chapter 8). The target is tracked by radar. The tracking radar itself may be used as the source of target illumination for missile guidance, or a separate radar may be used for this purpose.

9C3. Missile components

The missile, throughout its flight, is between the target and the radar that illuminates the target. It will receive radiation from the launching station, as well as reflections from the target. The missile must therefore have some means for distinguishing between the two signals, so that it can home on the target rather than on the launching station. This can be done in several ways. For example, a highly directional antenna may be mounted in the nose of the missile, as noted below. Or the doppler principle may be used to distinguish between the transmitter signal and the target echoes. Since the missile is receding from the transmitter, and approaching the target, the echo signals will be of a higher frequency.

For the purposes of this text, we can think of the missile guidance components as divided into several distinct sections. These are shown in block diagram form in figure 9C1.

ANTENNA. Radar is generally used for semiactive homing guidance. The antenna in

Figure 9C1.-Block diagram of semiactive homing system.

the missile must therefore be capable of detecting radiation at radar frequencies. It is mounted in the nose of the missile, since information is being obtained from the target area and the missile is approaching the target nose first. When a beam-riding system is used for the intermediate phase of guidance, a separate beam-rider antenna is mounted near the tail of the missile.

ANTENNA DRIVE. In some systems, the homing guidance antenna may use a form of conical (or nutating) scan in order to take full advantage of the guidance signal. Conical scanning has the advantage that the antenna can receive signals from points off the missile axis. This decreases the chance that the missile, while homing, may lose its target and go out of control.

RECEIVER. A radar-type receiver must be used in the missile when radar is used for semiactive homing. The signals picked up by the antenna as it scans the target area are fed into the receiver. The receiver operates in a conventional manner as described in chapter 7. The signals at the output of the receiver are not suitable for use in activating the missile flight controls without further processing.

SIGNAL CONVERTER. The receiver output is fed to the signal converter, which changes the signal to a form that can be used

REFERENCE UNIT. The reference unit furnishes the comparison signal. This information might be placed in the missile just prior to launching. It could be stored in a variety of forms, such as magnetic wires, magnetic tapes, punched paper tapes or punched cards. Before a guidance system can function, an error signal must be produced. The error in flight path, if any, can be determined by comparing the reference signal and the signal from the converter section. Comparison of the two signals takes place in other sections of the missile electronic equipment.

COMPARATORS. The missile flight controls may be used to correct the lateral or vertical trajectory of the missile. Since it is possible for the missile to be on the right course vertically but off course laterally, two comparators are used. The output from the reference unit and the output from the signal converter are fed to both the pitch and yaw comparators.

Should there be no difference in the two signals at either comparator, the controls would remain in neutral position. However, should there be a difference in the two signals at either comparator, error signals will be generated. The error signals are not suitable for use in controlling the missile flight surfaces and must be sent to other sections of the guidance system before they can be used.

AUTOPILOT. The missile flight control surface operation is controlled by autopilots. These devices are a combination of gyroscopes

9C4. Comparison with passive homing

The passive guidance system obtains all guidance information from the target, without assistance from any other outside source. The semiactive homing system needs some source outside the target or missile in order to obtain course information.

The advantage of the passive system is that it needs no source of information other than the target. The equipment carried by the missile is less than that required for most other systems. The disadvantage of the passive guidance system is its dependence on the target. It is highly unlikely that an enemy would leave target areas lighted, or permit electromagnetic forms of broadcasting from the target areas.

In the semiactive system, control information comes from a source outside the missile or target area. A semiactive homing system depends for guidance on equipment outside the target area or the missile. This requires extra equipment, both in the missile and at the launching or control point. Semiactive homing systems, like most guidance systems, are subject to jamming and other forms of interference.

The active guidance system uses equipment in the missile to illuminate the target, and to guide the missile to the target. Usually, a radar set is used for target illumination. The signals return to the missile as radar echoes, which are processed for use as guidance signals.

9D2. Missile components

The missile components in an active homing guidance system include all those used in a

ANTENNA. The antenna is the same as described for the semiactive system, and is mounted in the nose of the missile.

ANTENNA DRIVE. When the target area is conically scanned, the antenna driving unit provides the power needed for this purpose.

TRANSMITTER. The transmitter carried in the missile is similar to a conventional radar transmitter. It may use either FM or pulsed modulation. Since homing guidance does

Figure 9D1.-Block diagram of active homing guidance system.

not require long range equipment, the transmitter power can be considerably less than that used for command guidance or tracking. The method of modulating the transmitter can be changed frequently to lessen the effectiveness of enemy countermeasures.

DUPLEXER. The duplexer is a form of electronic switch. In operation, it serves to connect the antenna to the transmitter during the sending of a pulse. At the same time, it presents a high impedance (electrical opposition) at the receiver input. This keeps the powerful transmitter pulse from damaging the receiver.

As soon as the pulse is transmitted, the duplexer then offers a low impedance path from the antenna to the receiver. The action of the duplexer provides an automatic switching means, so that the same antenna can be used for both transmitting and receiving.